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This line graph has a Y axis labelled ‘relative signal amplitude’, measured in decibels, and has points minus 100 (bottom) and 0 (top) marked.

The X axis is labelled as frequency, measured in kilohertz. The scale is not linear, more of a logarithmic scale. From left to right it has points 0.02, 1, 5 and 20 marked.

A smooth analogue waveform is shown, starting at a position 0 on the Y axis. It is labelled ‘hearing sensitivity threshold’. It curves and drops smoothly to a trough at position 1 on the X axis and minus 100 on the Y axis. It remains at minus 100 until about position 5 on the X axis, then rises sharply (note also the logarithmic scale) to a maximum of 0 on the Y axis at position 20 on the X axis.

Two blue arrows labelled A and B are shown, starting at minus 100 on the Y axis, both pointing upwards. B is positioned just to the right of position 0.02 on the X axis. A is positioned to the right of B. The two arrows are of equal amplitude (height). B does not cross the hearing sensitivity threshold curve, but A does.

 3.4 Perceptual encoding